The manufacture of glass-ceramic articles involves three fundamental steps: first, a glass-forming batch, normally containing a nucleating agent, is melted; second, the melt is cooled at a sufficiently rapid rate that a glass body is formed which is essentially free of crystals; and, third, the glass body is exposed to a characteristic heat treatment procedure to cause crystallization in situ to take place.
Customarily, the third or crystallization step is performed in two parts. Thus, the glass body is initially heated to a temperature slightly above the transformation range thereof to cause the development of nuclei in the glass body which provide sites for the subsequent growth of crystals. Thereafter, the nucleated glass body is heated to a higher temperature, commonly above the softening point of the glass, to effect the growth of crystals on the nuclei.
The crystallization in situ mechanism leads to the substantially simultaneous growth of crystals on countless nuclei. Accordingly, a glass-ceramic article conventionally consists of uniformly fine-grained crystals randomly oriented, but homogeneously dispersed, throughout a residual glassy matrix. Normally, a glass-ceramic article is highly crystalline, i.e., at least 50% and frequently in excess of 75% by volume crystalline. This high crystallinity dictates that the physical properties exhibited by a glass-ceramic article are more nearly similar to those of the crystals than to those of the glassy phase. Furthermore, the composition of, and consequently the physical properties of, the residual glassy matrix are far removed from those of the parent or precursor glass since the constituents comprising the crystal phase will have been extracted therefrom. Finally, the crystallization in situ mechanism provides articles which are free from voids and non-porous.
U.S. Pat. No. 2,920,971 initiated glass-ceramic technology and reference is hereby made to that patent for a more detailed discussion of the microstructure, physical properties, and the method for making such articles.
U.S. Pat. No. 3,582,385 describes glass-ceramic bodies exhibiting good dimensional stability up to temperatures of about 800.degree. C. The bodies had compositions consisting essentially, by weight on the oxide basis, of 3.5-5% Li.sub.2 O, 2.5-5% BaO, 15-21% Al.sub.2 O.sub.3, 65-75% SiO.sub.2, and 3.5-8% of a nucleating agent composed of 3-8% TiO.sub.2 and 0-3% ZrO.sub.2, the sum of Li.sub.2 O, BaO, Al.sub.2 O.sub.3, SiO.sub.2, TiO.sub.2, and ZrO.sub.2 constituting at least 98% of the full composition. Beta-spodumene solid solution comprised the primary crystal phase with a minor amount of celsian. Oxides such as MgO, ZnO, and B.sub.2 O.sub.3 were preferably absent from the compositions because their presence led to the formation of secondary crystal phases having varying solid solubility with beta-spodumene. For example, the inclusion of MgO might lead to the growth of such secondary crystal phases as spinel, cordierite, and/or cristobalite either during the crystallization heat treatment or, more significant from a product standpoint, during subsequent prolonged exposures of the body to high temperatures. Cristobalite is a high expansion form of silica which often develops along with cordierite in thermally unstable, magnesia-containing, beta-spodumene solid solution glass-ceramic articles. The density changes that can accompany the growth of such phases will be reflected in overall dimensional instability of the article at elevated temperatures. The combined development of cordierite and cristobalite is particularly undesirable and will result in elongations of several thousand parts per million after relative brief exposures to a temperature of 950.degree. C. Strains of that level far exceed the strain tolerance of the bodies when utilized as fine-bore honeycombs in regenerative heat exchanger applications.
Ser. No. 578,379, filed May 19, 1975 by the present applicant, now U.S. Pat. No. 4,042,403, is also directed to glass-ceramic bodies demonstrating excellent high temperature dimensional stability. The operable compositions consist essentially, by weight on the oxide basis, of about 3-5% Li.sub.2 O, 0.25-2.5% MgO, 15-20% Al.sub.2 O.sub.3, 68-75% SiO.sub.2, and 2.5-5% TiO.sub.2 with, optionally, up to 3% ZrO.sub.2. A vital facet of the composition involves maintaining a molar ratio Li.sub.2 O:MgO of at least 2:1. Beta-spodumene solid solution comprises the principal and, sometimes, sole crystal phase with anatase and/or rutile frequently composing a secondary crystal phase. The Li.sub.2 O:MgO ratio is critical in assuring the absence of the development of cordierite and/or cristobalite as an extraneous crystal phase.